Future Broadband Satellite Communication Systems are targeting to approach the Terabit per second aggregated capacity [RD 1]. This target can be achieved thanks to the extensive reuse of frequency in multibeam systems. The deriving aggregated bandwidth is therefore extremely large, implying the deployment of a large network of Gateways, which is very costly.
A possible way forward to reduce the number of gateways required, and therefore minimise the ground segment cost, is the exploitation of higher frequency bands (e.g. Q/V band) where larger chunks of bandwidths are available.
As an instance, in Europe 4 GHz are available in Q/V band, against the 2 GHz in Ka band. The factor of 2 in the available bandwidth directly translates in halving the number of gateways required, therefore halving also the ground segment cost.
Moving the feeder link towards higher frequency bands such as Q/V band though is not costless. The impact of the atmospheric phenomena is much more critical than in Ka in terms of attenuation undergone by the signal when clouds or rain are present. An example comparing the total atmospheric attenuation (in dB) not exceeded for 99.9% of the time in the average year at 30 GHz (Ka band) and 50 GHz (V band) is shown in FIG. 1(a) and FIG. 1(b) respectively. The values are computed according to [RD 2]. As it can be seen from the figures, on the Feeder Uplink (forward link) the Atmospheric attenuation is <20 dB for 99.9% of the time in Ka (30 GHz), whereas in Q/V (50 GHz) it increases up to 45-50 dB.
The large fade dynamic at these frequencies though risks jeopardising the availability of the link itself whenever a meteorological phenomena such as rain occurs. The large fade dynamic requires system designers to modify some common techniques for mitigate fading events. Such techniques are commonly known as Fade Mitigation Techniques (FMT).
Uplink power control is a Fade Mitigation Technique which has been widely studied since the early 1970s, and which consists in changing the output power of a High Power Amplifier (HPA) to modify the transmitted power in order to achieve a given result.
In satellite communications, Uplink Power Control in fixed links at high frequencies (i.e. Ku or higher) has been traditionally applied in such a way to compensate all the extra attenuation due to an atmospheric phenomena with respect to the nominal attenuation experienced by the signal in clear sky [RD 10].
The system architecture of a multibeam star network is shown in FIG. 2. Every gateway (GWj, GWi, GWk)) deployed within the service area serves a subset of the User Beams. Every gateway transmits typically in the full bandwidth allocated to feeder links, and possibly on two orthogonal polarizations to minimise the required number of gateways required by the system thus reducing the ground segment cost. Lprop,CS denotes the clear sky propagation attenuation.
The gateways can reuse the same frequency thanks to spatial isolation of the multibeam feeder link antenna. Anyway, due to imperfect isolation, some co-channel interference will still be present on the link.
FIG. 3 instead shows the case where one of the gateways is under fading conditions due for instance to a rain event. Lprop,A denotes the propagation attenuation in these non-clear sky conditions.
In clear sky, on the transmission side the output power Pout(i) transmitted by the gateway i is given by:Pout(i)=Psat−OBOeff(i),  (1)
where Psat is the Saturated Power of the Gateway HPA and OBOeff(i) is the Output BackOff of gateway i.
The received power at the satellite in clear sky CCS(i) will be given then by:CCS(i)[dB]=EIRPsat[dBW]−OBOeff(i)[dB]−Lprop,CS(i)[dB]+Rx_Gi,i[dBi],  (2)where Lprop,CS(i) is the propagation losses of the signal transmitted by gateway i and Rx_Gi,i is the gain of the antenna beam i towards gateway i.
When a fading event occurs in the feeder link, without power control the received signal power at the satellite becomes:CA(i)[dB]=CCS(i)[dB]−A.  (3)
The Co-channel Interference of the gateway i is due to the portion of the power transmitted by the other gateways (k and j in FIG. 2) and received by beam i. The C/I cochannel in clear sky can be therefore quantified as:(C/Ico-channel)CS(i)[dB]=CCS(i)[dB]−(CCSij[dB]+CCSik[dB])  (4)where:CCSij[dB]=EIRPsat[dBW]−OBOeff(j)[dB]−Lprop,CS(j)[dB]+Rx_Gij[dBi]  (5)andCCSik[dB]=EIRPsat[dBW]−OBOeff(k)[dB]−Lprop,CS(k)[dB]+Rx_Gik[dBi].  (6)
When the gateway i undergoes a fading event (FIG. 3), the CCS(i) will be affected by the increased amount of Lprop(i) whereas the other gateways will most likely not be affected by the fading as due to the big distance the atmospheric conditions will be almost uncorrelated. As a consequence, the C/I cochannel will become:(C/Ico-channel)A(i)[dB]=(CCS(i)[dB]−A)−(CCSij[dB]+CCSik[dB]).  (7)
On the other hand, for the case of Uplink Power control, in clear sky the HPA will be operated at larger backoff (lower output power), and when the feeder link experience an attenuation A then the HPA is driven to a lower OBO to compensate A:CA(i)[dB]=CCS(i)[dB]−A+ULPCA,  (8)where ULPCA=min{A,ULPCrange} and ULPCrange is the dynamic range of the HPA.
Besides attenuating the fading due to excess propagation attenuation, this technique brings two major benefits: the intra system interference due to the frequency reuse in a multibeam system and due to the adjacent channels is kept constant as long as the extra attenuation does not exceed the Power Control Dynamic Range, and intermodulation products and distortions generated in the gateway due to the amplifier non-linearities are mitigated in clear sky. In fact, in nominal conditions a lower power is necessary. Working at lower power with a oversized HPA corresponds to increase the Input Back Off therefore operating in linear region.
FIGS. 4(a) and 4(b) illustrate how some key parameters of the uplink of a satellite communication system depend on uplink attenuation, with —FIG. 4(b)—and without—FIG. 4(a)—Uplink Power Control (PC). The relevant parameters are:                The Output Back Off (OBO), representing the difference between the saturated power at the output of the HPA and the actual transmitted power;        The Signal Power over noise ratio (SNR, or C/N);        The Noise Power Ratio (NPR) which indicates the ratio between the useful signal and the intermodulation products.        The Signal to Cochannel interference ratio (C/I cochannel);        The overall Signal Power over noise plus interference ratio C/(N+I), including the cumulated effect of noise, cochannel interference and intermodulation interference (as a preliminary analysis the effect of cross-polar interference has been neglected as it has a lower impact on the end-to-end performance).        
As it can be seen, without power control, in Clear Sky, all the power amplifier power is boosted. As soon as the link undergoes fading, the C/I co-channel and the C/N decrease linearly with it.
On the contrary, with Power Control the clear sky C/N is lower since assuming the same HPA sizing is done, less power is transmitted in nominal conditions. When the link undergoes an amount of fading attenuation A, the OBO of the HPA is reduced by A to keep a constant power at the satellite input. This allows keeping a constant C/I co-channel and C/N as long as the power control range is not exceeded (6 dB in the example).
Particular embodiments of “conventional” Uplink Power Control, implementing the general principles discussed above, are disclosed by the following prior art documents:                [RD 3] proposes a method to compensate slow varying fading due to rain based on statistical measurements of the power magnitude of the received reference signal.        [RD 4] describes a power control method in the uplink of a satellite communication system aiming to maintain the power level of the signal arriving at the satellite at the desired level.        [RD 5] proposes a power control technique in the uplink for a fixed-gain bent pipe transponder. In this kind of payload, the signal transmitted by the satellite is just an amplified and frequency converted version of the uplink signal. No level control is assumed to be implemented in the payload. The idea is to recover for the atmospheric attenuation in downlink Adown by increasing the transmitted power in uplink by the same quantity. This allows recovering the attenuation at the expenses of a small variation of the onboard HPA Output Back Off (OBO) implying therefore a degradation in terms of intermodulation products. The document proposes to operate the power control in such a way to recover the whole attenuation Adown.        [RD 6] describes a method to maintain the output power at a bent pipe satellite constant, fully compensating the uplink fade.        
Document [RD 7] describes a different UPC technique concatenating power control and channel coding. According to this method, the transmitted power on the uplink is adjusted based on the bit error rate (BER) measure done at the receiver side. When a high number of errors is measured, the system tries first to ask the transmitter to increase the transmitted power. Otherwise stated, the method aims at achieving constant BER, and not necessarily constant power at the satellite input.
[RD 8] describes a method and apparatus to compensate slow and fast variation of the power of the signal at the input of the satellite due to rain clouds and scintillation in order to keep a constant power at the input of the satellite.
[RD 9] presents a method for dynamically setting the operating point of an amplifier in a distributed meshed satellite network to avoid saturation. The adjustment is based on BER measures taken at receiving terminals from a signal transmitted at N power levels.